Time scaling refers to altering the time evolution or duration of an audio signal while not altering the spectral content (perceived timbre) or perceived pitch of the signal (where pitch is a characteristic associated with periodic audio signals). Pitch scaling refers to modifying the spectral content or perceived pitch of an audio signal while not affecting its time evolution or duration. Time scaling and pitch scaling are dual methods of one another. For example, a digitized audio signal's pitch may be scaled up by 5% without affecting its time duration by increasing the time duration of the signal by time scaling it by 5% and then reading out the samples at a 5% higher sample rate (e.g., by resampling), thereby maintaining its original time duration. The resulting signal has the same time duration as the original signal but with modified pitch or spectral characteristics. As discussed further below, resampling may be applied but is not an essential step unless it is desired to maintain a constant output sampling rate or to maintain the input and output sampling rates the same.
There are many uses for a high quality method that provides independent control of the time and pitch characteristics of an audio signal. This is particularly true for high fidelity, multichannel audio that may contain wide ranging content from simple tone signals to voice signals and complex musical passages. Uses for time and pitch scaling include audio/video broadcast, audio/video postproduction synchronization and multi-track audio recording and mixing. In the audio/video broadcast and post production environment it may be necessary to play back the video at a different rate from the source material, resulting in a pitch-scaled version of the accompanying audio signal. Pitch scaling the audio can maintain synchronization between the audio and video while preserving the timbre and pitch of the original source material. In multi-track audio or audio/video postproduction, it may be required for new material to match the time-constrained duration of an audio or video piece. Time-scaling the audio can time-constrain the new piece of audio without modifying the timbre and pitch of the source audio.